Remove obsolete initial-file-handling-state and clipboard-files attributes and document mandatory attributes

Allow globally overriding slots in the light dom of components and use it to override the auth-info slot in the Nextcloud file picker

color --dbp-override-downloaded-bg-color is now optional, because as default the primary button becomes an ordinary button (background-color: none)
Also mark all buttons when 'zip-download-button' is clicked

(CSS var in getThemeCSS() required)